1. Field of the Invention
This invention relates to a drawing device for use in a drafting machine, and more particularly to a drawing device arranged such that a lead can be thrust out continuously and sequentially and drawing operation can be conducted with a sufficient drawing pencil depressing pressure.
2. Description of the Prior Art
Drawing devices for use in drafting machines adapted to thrust a pencil lead automatically and sequentially are known, for example, from Japanese Patent Publication NO. SHO 57-36879, and Japanese Utility Model Publication NO. SHO 57-5193, etc..
Since such prior art drawing devices for use in drafting machines are not provided with any special mechanism to expel or remove the residual lead, it is necessary to conduct normal lead thrusting operations repeatedly to enable the residual lead to be pushed out by the next new lead. Therefore, the residual lead expelling operation per se is troublesome and takes a long time.
As a drawing device for a drafting machine which solves such a disadvantage, there is proposed an arrangement shown in Japanese Utility Model Laid-open Publication NO. SHO 62-41585.
This drafting device is arranged as shown in FIG. 6 such that a casing "b" provided at its leading end with a lead guide "a" is mounted on a base "c"; a pipe member "e" having on the leading end side thereof a chuck ring "d" fixedly secured thereto is inserted in the casing "b" so as to be vertically slidably moved freely; a lead stock "g" provided at its leading end with a lead chuck "f" is inserted in the pipe member "e" so as to be slidably moved freely; a coiled spring "h" is mounted between the lead stock "g" and the chuck ring "d" so as to bias and hold the lead chuck "f" at a position where it is radially deformed or contracted by the chuck ring "d", the base "c" is provided with a driving arrangement "i" for vertically moving the pipe member "e" and a stopper "j" for controlling the upward movement of the lead stock " g", the arrangement being made such that when the pipe member "e" is moved up and down by means of the driving arrangement "i" the lead can be thrust out sequentially, and when the pipe member "e" is moved further above a position where the lead stock "g" is abutted against the stopper "j" the lead chuck "f" is deformed so as to expand radially to thereby enable the next new lead to drop from the lead stock "g" and to abut against the residual lead.
In such a drawing device for use in a drafting machine, since lead thrusting operation is conducted by lowering the pipe member "e" so as to thrust a lead held by the lead chuck "f" out from a lead guide "a", and then raising the pipe member "e" to allow the lead chuck "f" to release the lead while the lead is held by the lead guide "a", if the lead holding force of the lead stock "f" is increased; that is, the resilient force of the coiled spring is increased, then upon upward movement of the pipe member "e", the lead chuck "f" does not release the lead and is moved upwards together with the lead held thereby, thereby rendering it unable to thrust the lead out. Therefore, the lead holding force of the lead chuck "f" is set to be somewhat weak.
Therefore, in case the lead powder deposits on the lead chuck "f" or the diameter of the lead is thinner than a predetermined value, there is a tendency of the lead moving upward relative to the lead chuck "f" by the action of the drawing pencil depressing pressure exerted on a drawing paper when drawing operation is conducted, thus rendering it unable to provide a sufficient drawing pencil depressing pressure.